Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier recording materials include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
For aqueous dye-based inks, the dyes needs to be sufficiently soluble in water to prepare a solution that is capable of producing adequate density on the receiving element and stable for extended periods of storage without precipitation. High quality ink jet printing with dye-based inks requires dyes which will provide both bright hue and good light fastness.
To generate full color prints via ink jet printing, ink sets comprising at least cyan, magenta and yellow inks are normally utilized. In addition a black ink is often added to enhance the printing of text and darker colors. The range of colors that can be produced with a given set of inks defines the color gamut of that ink set. For the production of high quality photorealistic images via ink jet printing, ink sets with a large color gamut are preferred. In addition, it is important that the ink sets produce images with good fastness, especially to light.
The choice of the colorants in ink jet systems is critical for both light fastness and color gamut. The color gamut of an ink set is controlled primarily by the spectral absorption characteristics of the component dyes. The primary dyes (e.g., cyan, magenta and yellow) should only absorb light of the required wavelengths (i.e., have relatively narrow absorption bands) and not overlap excessively with the dyes in the complementary inks. In addition to the light absorption characteristics of the dyes, the maximum print density achieved from the ink is another important factor to provide a high color gamut. The higher the print density the greater the color gamut.
Of particular importance is the light fastness of the component dyes when printed on recording materials which are subsequently sealed with a laminating film. Lamination is done to protect the printed image from physical damage and also to further enhance the stability of the printed image against light fade and fade due to environmental pollutants such as ozone.
Recording material laminate films are available in a variety of forms and can be categorized on their mode of adhesion to the printed recording material. These forms include heat activated laminates, thermal laminates and pressure sensitive laminates. Laminate films can contain ultraviolet light absorbing compounds which can in part protect the printed dyes from fade due to ultraviolet light.
Numerous dye based ink jet ink sets have been described in patent literature. For example, U.S. Pat. No. 6,152,969 relates to a set of inks comprising C.I. Direct Blue 199, C.I. Direct Yellow 86 and an anthrapyridone magenta dye however no mention is made of a suitable black dye for the ink jet ink set. U.S. Pat. No. 6,183,548 B1 related to a set of inks comprising Direct Yellow 107, Direct Yellow 132 or Direct Yellow 86, a water soluble, transition metal complex of an 8-hetercyclylazo-5-hydroquinoline dye, a sulfonated copper phthalocyanine dye and black dyes selected from Reactive Black 31, Direct Black 19, Direct Black 168 or Solubilized Sulfur Black 1. Patent Application EP 1,167,472 discloses an ink jet ink set comprising C.I. Direct Blue 199, Ilford M-377 magenta dye (CAS # 182061-89-8) in a magenta dye mixture with C.I. Acid Red 52, C.I. Direct Yellow 132 and C.I. Food Black 2.
U.S. Pat. No. 5,725,641 describes Reactive Black 31, a disazo metal (specifically copper) complex dye as unsuitable for ink jet ink compositions since at neutral pH it hydrolyzes over time causing the ink pH to drop and an increase in sulfonate ions. This can be detrimental to metal surfaces in the ink jet print head due to corrosion resulting from the acidic pH and increased ion content. Direct Black 19 exhibits severe bronzing (a metallic like reflection) on several types of ink jet ink receiving media. Direct Black 168 and Food Black 2 both exhibit unacceptable light fastness when compared to the other dyes in an ink jet ink set. U.S. Pat. No. 4,343,653 states that because of hydrolysis of the thiosulfonate salt group, Solubilized Sulfur Black 1 will slowly precipitate from the ink and will likely result in clogging of the print head nozzles. What is needed then is an ink jet ink set comprising cyan, magenta, yellow and black inks all having good light fastness on laminated media and without the physical and chemical problems of black dyes previously included in ink jet ink sets.
U.S. patent application Publication 2001/0027734A1 describes black trisazo metal complex dyes for use in ink jet ink sets but preferred cyan, magenta and yellow dyes are not described, in particular for their ability to provide acceptable light fastness on laminated ink jet receivers.
It is an object of this invention to provide an ink set capable of producing images with high light fastness when printed on laminated recording materials.